<!DOCTYPE html>
<html>
<head>
<style>
    body {
        margin: 0;
    }

    .absolute-box {
        position: absolute;
        top: 50px;
        height: 50px;
        width: 100%;
        background-color: green;
    }
</style>
</head>
<body>
  <div class="absolute-box"></div>
</body>
</html>
